Renting is about 30% more expensive in Seattle, and just about every other metric that you can measure costs of living by also favour Vancouver. The fact that Vancouver is on the west coast, and the fact that salaries are lower there than there are in the US is what is currently attracting more technology companies to open offices there. This isn't a theory, Vancouver has Canadas fastest growing tech sector. These are the same conditions that attracted companies to open offices further up the west coast to begin with, only now Seattle doesn't offer the same advantages that it used to.
Average price of a detached house in Vancouver is $1.5 million and avg software salary is C$72,945 per year (completely unaffordable). Average price of a detached house in Seattle is $772,729 and avg software salary is $110,907 a year. Still unaffordable and getting moreso, but doable with dual income.
The cost of buying a house and the cost of living are not the same thing. For instance the price to rent ratio in Vancouver is about twice as much as it is in Seattle. In any case all of this is completely irrelevant. The cost of living and the salaries are lower in Vancouver than Seattle, a fact that isn't up for debate. One of the core factors that have driven technology companies to expand beyond the valley is the cost of salaries, this is one of the reasons that the sector expanded so much in WA, and it's one of the key reasons that it is _currently_ expanding so much in B.C. None of this is theory, or me arguing for what I think will happen, it is what's happening right now.